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Transcript of Hydrogen Fuel - US Department of Energy...Hydrogen Fuel Cell Engines MODULE 2: HYDROGEN USE...

  • Hydrogen Fuel Cell Engines

    MODULE 2: HYDROGEN USE

    CONTENTS 2.1 HYDROGEN MANUFACTURE.................................................................................... 2-1

    2.1.1 ELECTROLYSIS ................................................................................................. 2-3 2.1.2 REFORMING ..................................................................................................... 2-9 2.1.3 THERMOCHEMICAL WATER DECOMPOSITION..................................................... 2-30 2.1.4 PHOTO CONVERSION ...................................................................................... 2-31 2.1.5 PRODUCTION FROM BIOMASS .......................................................................... 2-33 2.1.6 INDUSTRIAL PROCESSES ................................................................................. 2-34

    2.2 HYDROGEN STORAGE.......................................................................................... 2-36 2.2.1 HIGH PRESSURE GAS ..................................................................................... 2-37 2.2.2 LIQUID ........................................................................................................... 2-44 2.2.3 METAL HYDRIDES ........................................................................................... 2-46 2.2.4 OTHER STORAGE METHODS............................................................................ 2-51

    2.3 HYDROGEN TRANSPORTATION.............................................................................. 2-52 2.3.1 PIPELINES...................................................................................................... 2-52 2.3.2 MOBILE TRANSPORT ....................................................................................... 2-53 2.3.3 SITE MANUFACTURE ....................................................................................... 2-54

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    OBJECTIVES

    At the completion of this module, the technician will understand:

    • the processes by which hydrogen is extracted from other substances

    • how hydrogen is stored, and the inherent advantages and disadvantages of each method

    • how hydrogen is transported, and the inherent advantages and disadvantages of each method

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    Key Points & Notes 2.1 Hydrogen Manufacture The fundamental question underlying the use of hydrogen as a fuel is, Where do we get it from? Despite its abundance in the universe, hydrogen does not occur freely on earth, as it reacts very readily with other elements. For this reason, the vast majority of hydrogen is bound into molecular com-pounds.

    To obtain hydrogen means to remove it from these other molecules. With respect to the energy required, it is easy to remove hydrogen from compounds that are at a higher en-ergy state, such as fossil fuels. This process releases energy, reducing the amount of process energy required. It takes more energy to extract hydrogen from compounds that are at a lower energy state, such as water, as energy has to be added to the process.

    The process of extracting hydrogen from fossil fuels is called reforming. Today, this is the principal and least expensive method of producing hydrogen. Unfortunately, reforming emits pollutants and consumes non-renewable fuels.

    The process of extracting hydrogen from water is called elec-trolysis. In principal, electrolysis can be entirely non-polluting and renewable, but it requires the input of large amounts of electrical energy. Consequently, the total envi-ronmental impact of acquiring hydrogen through electrolysis is largely dependent on the impacts of the source power.

    One way to assess the comparative environmental impact of electrolysis and the reforming of various fuels is by compar-ing the total amount of carbon dioxide emitted. A recent study used a Mercedes-Benz A-Class car as the basis for comparison, and calculated the total carbon dioxide emis-sions per 1000 km of travel. The total carbon dioxide emis-sions include those from the car’s tailpipe, the fuel processor, and the powerplant that supplied the energy to the fuel processor.

    The results of the study, shown in Figure 2-1, clearly indi-cate that environmental cleanliness of hydrogen is directly linked to its mode of manufacture. Although not included in the study, hydrogen derived from renewable, non-fossil en-ergy sources would likely have no carbon dioxide emissions.

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    Key Points & Notes

    Figure 2-1 Total Carbon Dioxide Emissions for Comparative Hydrogen Production Methods

    Alternative methods of hydrogen production include thermo-chemical water decomposition, photoconversions, photobi-ological processes, production from biomass, and industrial processes. Although some of these methods show promise for the future, they are still largely experimental and capable of supplying only small amounts of hydrogen.

    Hydrogen can be produced on a large scale at dedicated hydrogen production plants, or on a small scale at local production facilities. Large-scale production benefits from economies of scale and plants can be located near power and water, but suffers from the difficulties of hydrogen transpor-tation. Some methods of hydrogen production, such as from coal or biomass, can only be undertaken on a large scale.

    Small scale production can reduce the problems of hydrogen transportation by using energy that is easily brought to the facility, such as electricity, natural gas or solar. On the downside, the amount of equipment required for the amount of hydrogen produced is significantly higher than for large-scale facilities, due to the economy of scale.

    At the extreme small scale, fossil fuels could be reformed to hydrogen on board a fuel cell vehicle, but the systems are complex and costly.

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    Key Points & Notes Currently, the vast majority of all hydrogen manufactured worldwide originates from fossil fuels, as a byproduct in chemical industries, or crude oil refining processes. Hydro-gen production from renewable energy is not yet feasible on a large scale.

    Production of hydrogen currently costs from 3 to 15 times more than natural gas, and from 1.5 to 9 times more than gasoline, depending on the method used. When extracted from fossil fuels, the initial production and refining of the fuel further increases the total cost.

    2.1.1 Electrolysis

    In electrolysis, electricity is used to decompose water into its elemental components: hydrogen and oxygen. Electrolysis is often touted as the preferred method of hydrogen production as it is the only process that need not rely on fossil fuels. It also has high product purity, and is feasible on small and large scales. Electrolysis can operate over a wide range of electrical energy capacities, for example, taking advantages of more abundant electricity at night.

    At the heart of electrolysis is an electrolyzer. An electrolyzer is a series of cells each with a positive and negative elec-trode. The electrodes are immersed in water that has been made electrically conductive, achieved by adding hydrogen or hydroxyl ions, usually in the form of alkaline potassium hydroxide (KOH).

    The anode (positive electrode) is typically made of nickel and copper and is coated with oxides of metals such as manga-nese, tungsten and ruthenium. The anode metals allow quick pairing of atomic oxygen into oxygen pairs at the elec-trode surface.

    The cathode (negative electrode) is typically made of nickel, coated with small quantities of platinum as a catalyst. The catalyst allows quick pairing of atomic hydrogen into pairs at the electrode surface and thereby increases the rate of hy-drogen production. Without the catalyst, atomic hydrogen would build up on the electrode and block current flow.

    A gas separator, or diaphragm, is used to prevent intermix-ing of the hydrogen and oxygen although it allows free pas-sage of ions. It is usually made of an asbestos-based material, and tends to break apart above 176 ºF (80 ºC).

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    Key Points & Notes

    Figure 2-2 Typical Electrolysis Cell

    The reactions at the cathode are:

    (1) K+ + e– K a positively charged potas-sium ion is reduced

    (2) K + H2O K+ + H + OH– the ion reacts with water to form a hydrogen atom and a hydroxyl ion

    (3) H + H H2 the highly reactive hydrogen atom then bonds to the metal of the cathode and combines with another bound hydrogen atom to form a hydrogen molecule that leaves the cathode as a gas

    The reactions at the anode are:

    (1) OH– OH + e– a negatively charged hy-droxyl ion is oxidized

    (2) OH ½H2O + ½O the ion reacts to form water and an oxygen atom

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    Key Points & Notes (3) O + O O2 the highly reactive oxygen

    atom then bonds to the metal of the anode and com-bines with another bound oxygen atom to form an oxygen molecule that leaves the anode as a gas

    The rate of hydrogen generation is related to the current density (the amount of current divided by the electrode area measured in amps per area). In general, the higher the cur-rent density, the higher the source voltage required, and the higher the power cost per unit of hydrogen. However, higher voltages decrease the overall size of the electrolyzer and therefore result in a lower capital cost. State-of-the-art elec-trolyzers are reliable, have energy efficiencies of 65 to 80% and operate at current densities of about 186 A/ft2 (2000 A/m2).

    Figure 2-3 Electrolyzers

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    Key Points & Notes A fuel cell reverses the process of electrolysis. Electrolysis adds electrical energy to low-energy water to release two high-energy gases. A fuel cell allows the gases to react and combine to form water, releasing electrical power. Both proc-esses release heat, which represents an energy loss.

    For electrolysis, the amount of electrical energy required can be somewhat offset by adding heat energy to the reaction. The minimum amount of voltage required to decompose water is 1.23 V at 77 ºF (25 ºC). At this voltage, the reaction requires heat energy from the outside to proceed. At 1.47 V (and same temperature) no input heat is required. At greater voltages (and same temperature) heat is released into the surroundings during water decomposition.

    Operating the electrolyzer at lower voltages with added heat is advantageous, as heat energy is usually cheaper than electricity, and can be recirculated within the process. Fur-thermore, the efficiency of the electrolysis increases with increased operating temperature.

    When viewed together with fuel cells, hydrogen produced through electrolysis can be seen as a way of storing electrical energy as a gas until it is needed. Hydrogen produced by electrolysis is therefore the energy carrier, not the energy source. The energy source derives from an external power generating plant. In this sense, the process of electrolysis is not very different from charging a battery, which also stores electrical energy. Viewed as an electricity storage medium, hydrogen is competitive with batteries in terms of weight and cost.

    To be truly clean, the electrical power stored during elec-trolysis must derive from non-polluting, renewable sources. If the power is derived from natural gas or coal, the pollution has not been eliminated, only pushed upstream. In addition, every energy transformation has an associate energy loss. Consequently, fossil fuels may be used with greater effi-ciency by means other than by driving the electrolysis of hydrogen. Furthermore, the cost of burning fossil fuels to generate electricity for electrolysis is three to five times that of reforming the hydrogen directly from the fossil fuel.

    The approximate efficiencies of modern thermal power stations are: Nuclear: 30–33% Natural gas: 30–40% Coal: 33–38% Oil: 34–40%

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    Key Points & Notes

    Figure 2-4 Energy Transformation Processes Based on Fossil Fuels

    Non-polluting renewable energy sources include hydroelec-tric, solar photovoltaic, solar thermal and wind. These meth-ods of power generation are applicable only in specific geographic or climatic conditions. Furthermore, with the exception of hydroelectric, each of these power sources is intermittent. Despite growth in the use of these energy sources, they currently provide a very small amount of the power consumed today.

    Hydroelectric power generation uses the energy of moving water to turn turbines that in turn rotate generators. Hy-droelectric power is only feasible in areas with major rivers that undergo significant changes in height. Most suitable locations worldwide have already been developed. Hydro-electric power is a cheap source of clean power especially when utilizing excess, off-peak power. The efficiency of hy-droelectric power generation can top 80%. This is probably the optimum form of renewable energy although the envi-ronmental and ecological cost of dams is high.

    Solar electric power generation uses banks of solar cells to convert the energy of the sun directly into electrical power. Solar power is only feasible in areas with significant amounts of intense sunlight and requires large tracts of land to generate sufficient levels of power. The efficiency of solar cells currently ranges from 3 to 17%.

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    Key Points & Notes

    This solar cell array contains tracking and flat cells which can generate a combined rated total of 50 HP (37 kW) in bright sunlight. The power is supplied at 360 VDC with a maximum current of 103 A.

    Figure 2-5 Solar Cell Array

    Solar thermal power generation starts by concentrating the sun’s heat into a fluid that has a high heat-carrying capac-ity, such as oil. The oil’s heat is transferred through heat exchangers to water, generating steam for a steam turbine generator. Like solar electric power generation, solar thermal power generation is only feasible in areas with significant amounts of intense sunlight and requires large tracts of land in order to generate sufficient levels of power. The overall efficiency of converting sunlight to electricity for these sys-tems is about 8 to 24% depending on the type of technology used.

    Wind power generation uses the energy of moving air to turn turbines that in turn rotate generators. Wind power is only feasible in areas with favorable wind conditions and requires large tracts of land in order to generate sufficient levels of power. Wind has low energy density and wind turbines oper-ating at optimum conditions (design speed) may obtain 30% efficiency at best. Real-world operating conditions may re-duce this efficiency considerably.

    Although a renewable energy source in conjunction with electrolysis would eliminate the dependence on fossil fuels, it does not reduce the number of energy transformations re-quired to produce mechanical work using hydrogen. If clean, renewable power were available, it could also be used in other ways that require fewer energy transformations, such as direct storage in batteries or to compress air for propul-sion.

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    Key Points & Notes Another consideration associated with electrolysis is the source of water. Water is already a precious commodity, and would be consumed in vast quantities in order to support a large hydrogen economy. The water would also have to be purified prior to use, increasing its cost.

    2.1.2 Reforming

    Reforming is a chemical process that reacts hydrogen-containing fuels in the presence of steam, oxygen, or both into a hydrogen-rich gas stream. When applied to solid fuels the reforming process is called gasification. The resulting hydrogen-rich gas mixture is called reformate. The equip-ment used to produce reformate is known as a reformer or fuel processor.

    The specific composition of the reformate depends on the source fuel and the process used, but it always contains other compounds such as nitrogen, carbon dioxide, carbon monoxide and some of the unreacted source fuel. When hydrogen is removed from the reformate, the remaining gas mixture is called raffinate.

    In essence, reforming a fossil fuel consists of the following steps:

    1. Feedstock purification (including sulfur removal).

    2. Steam reforming or oxidation of feedstock to form hydro-gen and carbon oxides.

    3. Primary purification—conversion of carbon monoxide to carbon dioxide.

    4. Secondary purification—further reduction of carbon monoxide.

    The reforming reactions require the input of water and heat. Overall reformer thermal efficiency is calculated as the LHV of the product hydrogen divided by the LHV of the total input fuel. This thermal efficiency depends on the efficiencies of the individual processes, the effectiveness to which heat can be transferred from one process to another, and the amount of energy that can be recovered through means such as turbochargers. In the end, high temperature reformer effi-ciencies are approximately 65% and low temperature methanol reformers can achieve 70 to 75%.

    The advantages of reforming fossil fuels are that they:

    • uses existing fuel infrastructures

    • reduces the need to transport and store hydrogen

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    Key Points & Notes • does not need the input of large amounts of energy as in electrolysis

    • is less expensive than other hydrogen production meth-ods

    The disadvantages of reformers are that they:

    • can have relatively long warm-up times

    • are difficult to apply to vehicle engines because of irregu-lar demands for power (transient response)

    • are complex, large and expensive

    • introduce additional losses into the energy conversion process, especially those that have small thermal mass

    • use non-renewable fossil fuels

    • generate pollution

    The pollution generated by reformers take three forms:

    • carbon dioxide emissions

    • incomplete reactions, leaving carbon monoxide and some of the source fuel in the reformate

    • production of pollutants through combustion, such as nitrous oxides

    Reforming fossil fuels only makes sense if the hydrogen is needed directly, as in a fuel cell engine. For internal combus-tion engines, it is always more efficient to use the fossil fuel directly without passing it through a reformer first.

    Medium- or large-size reformers can be installed at fuel cell vehicle fueling stations. At these scales, the equipment com-plexity, warmup time and transient response are not issues, pollutants can be controlled more effectively, and existing power infrastructures can be used. The facility must store only small amounts of hydrogen, and hydrogen transporta-tion is avoided.

    Small-size reformers can be installed in fuel cell vehicles to entirely eliminate the problems associated with fueling, storing and handling hydrogen directly. In fact, many fuel cell experts think that the true challenge in fuel cell engine design now lies in the development of an efficient, compact, reliable and highly integrated fuel processor. Other experts think that the use of on-board reformers will never pose a realistic solution due to their size, complexity and cost.

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    Key Points & Notes

    This natural gas reformer uses a partial oxidation reation, a water/gas shift reaction, and pressure swing adsorption techniques to generate up to 4,200 sft3/hr (1,980 slpm) of hydrogen. The hydrogen purity is 99.999%. Maximum water consumption is 4,000 gal/day (15,100 L/day).

    Figure 2-6 Medium-Scale Natural Gas Reformer

    2.1.2.1 Potential Reforming Fuels

    In theory, any hydrocarbon or alcohol fuel can serve as a feedstock to the reforming process. Naturally, fuels with existing distribution infrastructures are the most commonly used.

    Methane (Natural Gas)

    Natural gas has a well-established infrastructure and is the most economical of all reforming feedstocks. Natural gas contains low levels of sulfur compounds (primarily mercap-tans) that must be removed, as they would block active cata-lyst sites in the reformer and fuel cells. These sulfur compounds requires fuel purification (hydro-desulfurization) prior to reforming.

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    Key Points & Notes

    Figure 2-7 Typical Fuel Cell Process Schematic with Integral Natural Gas Stationary Reformer

    Since methane is a gas at atmospheric conditions, it cannot be used to advantage on an on-board vehicular fuel proces-sor, although it is an excellent source for stationary fuel reformers. Propane has similar reforming characteristics to natural gas.

    Methanol

    Methanol is readily available and is a favorite for on-board vehicular fuel processors as it can be reformed at relatively low temperatures of 390 to 570 ºF (200 to 300 ºC). However, methanol has lower energy content and energy density than liquid hydrocarbon fuels.

    Gasoline and Diesel

    Gasoline and diesel have very well established infrastruc-tures but have a lower hydrogen-to-carbon ratio than other feedstocks. This results in a lower hydrogen yield relative to the amount of waste gas. In general, heavier hydrocarbons

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    Key Points & Notes also contain sulfur trapped in long chains and cyclic com-pounds that typically require higher processing pressures of 90 to 650 psig (6 to 45 barg) to remove.

    In addition, petroleum products include compounds called olefins and aromatics that result from the refining process. Olefins are highly reactive and tend to form polymer gums or carbon. Aromatics are very stable and difficult to reform and may form carbon. Both gum and carbon formation block active catalyst sites and their formation are highly depend-ent on processing temperature. The presence of sulfur, olefin and aromatic compounds requires fuel purification and pre-reforming processes prior to reforming.

    Coal

    Coal has enormous supply potential but suffers from high impurity levels, low hydrogen yield, and is difficult to handle. Coal could only be reformed at large dedicated facilities and therefore faces the problems associated with transporting and storing hydrogen. Currently, coal reforming is the most expensive of all reforming methods.

    Reformate Composition Comparison

    Typical reformate compositions are indicated in Table 2-1. For each source fuel, the yield is calculated based on both steam reforming and partial oxidation (POX) reforming tech-niques. In both cases, the resulting carbon monoxide is then converted to carbon dioxide using a water/gas shift reaction followed by a selective oxidation reaction. (These techniques are described in the sections that follow.)

    Methane Methanol Gasoline/Diesel Coal

    Steam POX Steam POX Steam POX Steam POX

    Hydrogen 75.7% 47.3% 71.1% 37.8% 71.1% 37.8% 63.1% 23.6% Nitrogen 1.9% 33.5% 1.9% 39.8% 1.9% 39.8% 1.9% 49.2% Carbon Dioxide

    19.9% 16.7% 24.5% 19.9% 24.5% 19.9% 32.5% 24.7%

    Source Fuel/Other

    2.5% 2.5% 2.5% 2.5% 2.5% 2.5% 2.5% 2.5%

    Table 2-1 Typical Reformate Compositions

    All oxygen is added in the form of air. Any excess water used in the reforming or purification reactions may contribute to the humidification of the gas mixture, but is not indicated as a portion of the overall mixture.

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    Key Points & Notes Reformate can be used directly in (PEM) fuel cells instead of pure hydrogen although this has five important implications:

    • Firstly, the carbon monoxide and source fuel impurities must be reduced to acceptable levels, as they are fuel cell poisons.

    • Secondly, any unreacted liquid source fuel that passes into the fuel cells obstructs the power generation reac-tion.

    • Thirdly, although nitrogen, carbon dioxide and gaseous source fuels pass through the fuel cells without detri-mental effect, they dilute the amount of hydrogen present so that the fuel cell stacks and affiliated system equip-ment must be larger than if running on pure hydrogen.

    • Fourthly, the presence of gases other than hydrogen precludes the use of the fuel in a “dead-ended” manner: pure hydrogen can be recirculated through a fuel cell un-til entirely consumed, whereas reformate must pass through and be vented to prevent the steady accumula-tion of non-reactive gases. Since fuel cells must operate with more fuel than is stoichiometrically required, this results in direct hydrogen wastage with an associated overall loss in efficiency.

    • Fifthly, use of reformate usually eliminates the need for a fuel humidifier since the reforming process leaves the gas saturated with water.

    2.1.2.2 Types of Reformers

    Reformers are of three basic types: steam reformers, partial oxidation reactors and thermal decomposition reactors. A fourth type results from the combination of partial oxidation and steam reforming in a single reactor, called an autother-mal reformer.

    Steam Reformers

    Steam reformers are currently the most efficient, economical and widely used technique of hydrogen production. Steam reforming is based on the principal that hydrogen-containing fuels decompose in the presence of steam over nickel-based catalysts to produce a mixture of hydrogen and carbon mon-oxide. The steam reforming process is illustrated schemati-cally in Figure 2-8. The overall reaction for generic hydrocarbons is:

    When applied to methane, the overall reaction reduces to: CH4 + H2O + heat CO2 + 2 H2

    For methanol: CH3OH + H2O + heat CO2 + 3 H2 CnHm + n H2O + heat n CO + (n + m/2) H2

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    Key Points & Notes The output products also contain some unreacted source fuel and water. In addition, only light hydrocarbons can be completely vaporized without leaving a carbon residue. The carbon monoxide must be converted to carbon dioxide using supplementary processes (Section 2.1.2.3).

    A great benefit of steam reforming is that the hydrogen pre-sent in the water is released during the reaction and con-tributes to the overall hydrogen yield.

    The steam reforming process typically requires temperatures of 840 to 1700 ºF (450 °C to 925 °C) and pressures of ap-proximately 290 to 500 psig (20 to 35 barg). These tempera-tures are achieved through combustion of a portion of the reformate. These flame temperatures are too low to form nitrous oxides, which begin forming at temperatures above 2700 ºF (1480 ºC).

    Since the overall steam reforming reaction is endothermic (uses heat), the bulk of the heat energy can be recovered by feeding it back into the process. The extent to which the heat energy is recovered determines the thermal efficiency of the reformer, and can be as high as 85%. This high thermal efficiency is the primary feature of steam reformers.

    Thermal efficiencies can reach 90% when steam reforming methanol. This is because the decomposition reaction occurs at the much lower temperatures of 390 to 570 ºF (200 to 300 ºC), decreasing thermal losses accordingly.

    In addition to temperature and pressure, the fuel-to-water ratio plays a significant role in promoting the formation of hydrogen while suppressing undesired reactions. This water must either be drawn from an external source, or, if used in conjunction with a fuel cell, drawn from the fuel cell product water stream.

    The complex interactions between the fuel-to-water ratio, available heat, thermal mass and hydrogen demand result in relatively slow startup characteristics and poor response to transient demands for steam reformers. As a result, steam reformers for automotive applications require complex con-trol systems and creative engineering practices. Designs must be optimized to provide high surface area for heat transfer, high thermal inertia during transients and compact reactor dimensions.

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    Key Points & Notes

    Figure 2-8 Steam Reforming Process

    Steam reformers are not inherently compact. For a typical car, a methanol steam reformer would need a catalyst vol-ume of 1.3 to 1.8 gallons (5 to 7 L) and a tank volume for the methanol/water mixture about three times the size of a comparable gasoline or diesel tank.

    Partial Oxidation Reformers

    Partial oxidation reformers (or “POX” reformers) react a lean mixture of oxygen (air) with fuel to produce a mixture of hydrogen and carbon monoxide. The partial oxidation proc-ess is illustrated schematically in Figure 2-9. The overall reaction for generic hydrocarbons is:

    CnHm + n/2 O2 n CO + m/2 H2 + heat

    Since partial oxidation reformers use oxygen from air, nitro-gen passes through the reactor along with the reaction products, thereby diluting the fuel stream. The output prod-ucts also contain some unreacted source fuel. The carbon monoxide must be converted to carbon dioxide using sup-plementary processes (Section 2.1.2.3).

    Unlike stream reformers, partial oxidation reformers are typically used to reform heavier hydrocarbons such as gaso-line, diesel and heavy oil. A form of partial oxidation is used to gasify coal although the presence of sulfur and large amounts of ash, both of which must be removed, further complicates this process. These processes do not use cata-lysts and occur at 2100 to 2400 ºF (1150 to 1315 ºC) and on

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    Key Points & Notes the order of 880 psig (60 barg). Lighter hydrocarbons, such as methane, can be partially oxidized using catalysts at 1090 ºF (590 ºC).

    Figure 2-9 Partial Oxidation Process

    Partial oxidation reformers offer both disadvantages and advantages compared to stream reformers. The key disad-vantages of partial oxidation reformers are higher operating temperatures and pressures, lower thermal efficiency and lower hydrogen yield. The lower thermal efficiency stems primarily from the fact that the oxidation process in exo-thermic (releases heat). This means that a significant portion of the inlet fuel energy ends up as heat, which cannot be utilized by the reactor itself, nor by the fuel cell. The lower yield results from the fact that additional hydrogen is not added to the reaction in the form of water. The hydrogen yield is also diluted by large amounts of nitrogen when the oxygen is added in the form of air.

    Larger heat rejection requirements also result in higher parasitic cooling losses. In a pressurized partial oxidation reactor, some of the waste energy can be recovered using a burner/turboexpander combination to help pressurize the fuel cell process air. Nonetheless, exothermic partial oxida-tion reformers always face an overall efficiency handicap of 5 to 10% compared to endothermic steam reformers.

    The effect of increased hydrogen dilution (due to the pres-ence of nitrogen) increases the size of supplemental refor-

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    Key Points & Notes mate purification equipment, and requires a larger fuel cell stack for a given power output.

    The key advantages of partial oxidation reformers are their relative compactness, and potentially good startup and load-following characteristics. The improvement in these areas is related to the exothermic nature of the oxidation reaction — there is no time lag for heat transfer from external sources. Furthermore, partial oxidation processes offer the potential to use readily available gasoline and diesel supplies.

    Autothermal Reformers

    Autothermal reformers attempt to combine the compactness and load following capabilities of partial oxidation reactors with the efficiency of steam reformers by combining the two reactors in one unit. The autothermal process is illustrated schematically in Figure 2-10. The overall reactions for ge-neric hydrocarbons are:

    CnHm + n/2 O2 n CO + m/2 H2 + heat

    CnHm + n H2O + heat n CO + (n + m/2) H2

    Figure 2-10 Autothermal Process

    In an autothermal reformer, fuel, steam and oxygen (or air) are fed over a mixed catalyst bed that supports both partial oxidation and steam reforming reactions. The heat generated

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    Key Points & Notes by the partial oxidation reaction provides the heat required for the steam reforming reaction, and removes the need for an external burner or heat source. This process requires careful thermal integration and tight controls to ensure heat balance and temperature matching between the two reac-tions.

    While autothermal reformers overcome some of the efficiency limitations of partial oxidation reformers, they still operate at efficiencies below those of steam reformers and result in a dilute hydrogen stream. Furthermore, autothermal reformers are the least developed of all reformer systems and still re-quire significant engineering effort before they become prac-tical.

    Thermal Decomposition Reformers

    Thermal decomposition reformers (or “catalytic crackers”) use heat to break down source fuels yielding high purity hydrogen (>95%) and solid carbon. The thermal decomposi-tion process is illustrated schematically in Figure 2-11. The overall reaction for generic hydrocarbons is:

    CnHm + heat n C + m/2 H2

    Figure 2-11 Thermal Decomposition Process

    Thermal decomposition reformers are very compact with quick startup and load-following characteristics but have the lowest thermal efficiency of any of the reforming systems (55–65%). The thermal efficiency is so low partly because a

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    Key Points & Notes great deal of the source fuel’s energy remains trapped in the product carbon (rather than in the lower energy forms of carbon monoxide or carbon dioxide). Efficiencies would im-prove significantly if the product carbon could be burned for heat generation, but this would lead to undesirable emission levels of nitrous oxides and would substantially increase system complexity.

    2.1.2.3 Reformate Purification

    Steam, partial oxidation and autothermal reformers all con-vert the carbon contained in the source fuel into carbon monoxide. Depending on the feedstock, the reformate stream can also include sulfur compounds, liquid methanol other contaminants. All of these compounds poison or degrade fuel cell performance and must be removed to very low levels. For example, alkaline fuel cells can tolerate no more than 3% (by volume) carbon monoxide, and PEM fuel cells can tolerate no more than 50 ppm carbon monoxide. Other reformate gases, such as nitrogen and carbon dioxide, dilute fuel cell per-formance but do not damage the cells.

    Reformate purification is a two-stage process. In the first stage, the bulk of the carbon monoxide is transformed into carbon dioxide using a water/gas shift reaction. In the sec-ond stage, the amount of carbon monoxide is further re-duced using selective oxidation or methanation reactions, and/or the hydrogen is extracted from the reformate stream using a pressure swing adsorption process or by means of metal separation membranes.

    The amount of reformate purification chosen is a trade-off between fuel cell longevity and performance, and overall system complexity, size and cost.

    Water/Gas Shift Reaction

    The water/gas shift reaction reacts carbon monoxide with steam over a catalyst to produce a mixture of hydrogen and carbon dioxide. The shift reaction process is illustrated schematically in Figure 2-12. The overall reaction is:

    CO + H2O CO2 + H2 + heat

    The water/gas shift reaction can be performed using high-temperature catalysts that support the reaction in excess of 570 ºF (300 ºC), or using low-temperature catalysts that support the reaction to about 285 ºF (150 ºC).

    The water/gas shift reaction is an exothermic process; this heat can be recovered and used in other parts of the reform-

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    Key Points & Notes ing process to improve the overall thermal efficiency. Some shift reaction catalysts also oxidize exothermically when exposed to air. This can result in high temperatures that can pose a fire hazard and/or damage the catalyst.

    Figure 2-12 Water/Gas Shift Reaction Process

    Like in a steam reformer, the water for the shift reaction must either be drawn from an external source, or if used in conjunction with a fuel cell, drawn from the fuel cell product water stream.

    The water/gas shift reaction is an efficient method of elimi-nating the bulk of carbon monoxide in a reformate stream. Carbon monoxide typically comprises between 15 and 60% of the reformate stream, depending on the feedstock used. The water/gas shift reaction can reduce this to levels less than 1%, and even as low as 0.2%.

    Selective Oxidation

    Selective oxidation (or “selox”) is a chemical process that reacts carbon monoxide with oxygen (air) over a catalyst to produce carbon dioxide. The selective oxidation process is illustrated schematically in Figure 2-13. The catalyst bed facilitates both of the following competing reactions:

    CO + ½O2 CO2 + heat

    H2 + ½O2 H2O+ heat

    The former reaction is preferred and is selected by control-ling the temperature profile within the selective oxidizer. Temperatures that are too high favor the water-producing

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    Key Points & Notes reaction whereas temperatures that are too low result in condensation. The selective oxidation reaction is an exo-thermic process; this heat can be recovered and used in other parts of the reforming process to improve the overall thermal efficiency.

    Figure 2-13 Selective Oxidation Process

    Selective oxidation is the most compact of all purification methods and is capable of reducing carbon monoxide con-tent to very low levels, such as 5 to 10 ppm. The main draw-back of selective oxidizers is that they require complex control systems, especially to maintain low carbon monoxide levels during load transients.

    Methanation

    Methanation is a chemical process that reacts carbon mon-oxide and carbon dioxide with hydrogen to produce methane and water. The methanation process is illustrated schemati-cally in Figure 2-14. The overall reactions are:

    CO + 3 H2 CH4 + H2O + heat

    CO2 + 4 H2 CH4 + 2 H2O + heat

    These reactions are the opposite of those that occur during steam reforming of methane and are therefore reversing the hydrogen production initially accomplished. If this process were performed on the reformate stream directly, virtually all of the product hydrogen would be consumed with no net yield. However, if the carbon dioxide is removed prior to methanation using some other means (such as pressure

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    Key Points & Notes swing adsorption), the remaining carbon monoxide can be successfully reduced to very low levels with little loss in overall hydrogen yield.

    When reforming methanol, the methanation process has the side benefit of simultaneously converting previously unre-acted methanol into carbon dioxide, hydrogen or methane. This not only increases the hydrogen yield, but also reduces the chance of liquid methanol entering the fuel cells where it can obstruct the power generation reaction.

    Figure 2-14 Methanation Process

    Pressure Swing Adsorption (PSA)

    Pressure swing adsorption (or “PSA”) is a common industrial method of separating a gas mixture into its various compo-nents. This technology has been used extensively in combi-nation with steam reformers to extract hydrogen on an industrial scale. Very high purity is attainable (99.99%) while recovering up to 85% of the available hydrogen.

    At the heart of the pressure swing adsorption process are materials called zeolites. A zeolite is a mineral with a highly structured crystalline surface that attracts gas molecules to its surface and holds them there through physical adsorp-tion (adsorb means “to stick to the surface of”. This is differ-ent from absorb that means “to penetrate into the surface of”). Many different types of zeolites are manufactured com-mercially, each capable of selectively adsorbing specific gases or groups of gases. Combinations of zeolites can there-fore be tailored to form “molecular sieves” capable of trap-ping unwanted gases while allowing desired gases to pass

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    Key Points & Notes unaffected. Pressure swing adsorption devices are routinely built to separate oxygen from air in the same manner that hydrogen is separated from fuel.

    Physical adsorption involves relatively weak “van der Waals” and electrostatic forces. These bonds are easily broken when subjected to pressure and flow variations. This principle is used to flush the trapped unwanted gases out of the zeolites after the purified product has been removed. The pressure difference between the initial gas mixture and the exhaust stream drives the process; hence the term pressure swing adsorption. Repeated cycles result in a continuous flow of purified product.

    Specifically, the pressure swing adsorption process involves four primary phases that together make up a complete cycle. During the first, or “feed pressurization” phase, reformate gas is introduced to the zeolite bed at high pressure. During the second, or “production” phase, the unwanted gases are adsorbed by the zeolites while purified hydrogen concen-trates at the top of the bed and is drawn off. During the third, or “depressurization” phase, the pressure is reduced and some of the unwanted gases are expelled. During the fourth, or “low pressure purge” phase, some of the pure hydrogen gas is back-flushed through the bed at low pres-sure in order to desorb and purge out the remaining un-wanted gases. The pressure swing adsorption process is illustrated schematically in Figure 2-15.

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    Figure 2-15 Pressure Swing Adsorption Process

    Conventional pressure swing adsorption machines use sta-tionary beds filled with small porous pellets formed of zeolite crystals mixed with a clay binder. These pellets can be as small as 0.5 mm in diameter but are more typically 2 or 3 mm in diameter. This configuration results in a very high surface area and therefore large adsorption capacity, but the size and arrangement of the pellets inhibits rapid gas trans-fer in and out of the pellets.

    When operated quickly, the pellet arrangement results in a high pressure drop, fluidization (the pellets lift with the gas and act like a fluid), and attrition (the pellets rub together, break apart, and disappear). To compensate, conventional machines must operate at very low speed (typically at 0.5 cycles/min) and tend to be large. However, new develop-ments in pressure swing adsorption technology use a pro-prietary adsorbent structure and novel rotary valve technology that allow two orders of magnitude increase in the gas separation cycle speed without fluidization or attri-tion. This allows the construction of devices that are a frac-tion of the size of conventional units with the potential for use on-board vehicles.

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    Key Points & Notes

    Figure 2-16 QuestAir Technologies’ State-of-the-Art Compact Pressure Swing Adsorption Device

    The greatest problem associated with using pressure swing adsorption in conjunction with fuel cells is the fact that the gases that come into contact with the zeolites must be com-pletely dry. This is because water adsorbs onto the surface of the zeolites and forms strong bonds that involve electron transfer and other strong forces. This is known as chemical adsorption, and is different from the physical adsorption that occurs with the waste gases.

    Chemical adsorption bonds are too strong to be broken us-ing pressure and flow fluctuations. As a result, these bonds displace potential physical adsorption bonds and render the zeolite bed less effective, ultimately disrupting the pressure swing adsorption process. Once formed, chemical adsorption bonds require heat to be broken. To prevent these bonds from forming, a desiccant must be used to adsorb water from the incoming wet gas stream before it comes into con-tact with the zeolites. This can be done concurrent with, and within the same machine as, the gas separation process so that there are two pressure swing adsorption cycles operat-ing — one for water separation and the other for hydrogen.

    Metal Separation Membranes

    Metal separation membranes present an impermeable physi-cal barrier to all gases except hydrogen.

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    Key Points & Notes To function, the reformate must be delivered to the mem-brane across a pressure gradient. Up to 85% of the hydrogen then diffuses from the high pressure (300 psig; 20 barg) reformate stream to form a low-pressure (30 psig; 2 barg) stream of very high purity (>99.999%). The remaining high-pressure raffinate can be combusted to provide heat for the reforming process. The metal separation membrane process is illustrated schematically in Figure 2-17.

    The advantages of metal separation membranes are that they provide high purity, undiluted hydrogen in a compact, simple and reliable manner. Metal separation membranes are also well suited to thermal integration with steam re-formers and respond well to thermal transients.

    The primary disadvantage of metal separation membranes is their very high material and construction costs. Tradition-ally, metal separation membranes are the most costly of all purification techniques, owing to their use of silver palla-dium or other alloys and the need to eliminate all pinholes and sealing leaks. However, new developments using thin film membranes promise to lower the materials cost to prac-tical levels.

    Other drawbacks to metal separation membranes include the need to re-humidify the hydrogen prior to use by fuel cells, and the need for a pressurized reformer. The use of a pressurized reformer results in increased reformer weight and size, although this is offset somewhat by the elimination of other purification equipment.

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    Key Points & Notes

    Figure 2-17 Metal Separation Membrane Process

    2.1.2.4 Heat Generation

    Heat is an essential part of any reforming process. For endo-thermic reactions, such as steam reforming and thermal decomposition, heat must be supplied from an external source. For exothermic reactions, such as partial oxidation and autothermal reactions, heat is generated internally by the reaction itself.

    The water/gas shift, selective oxidation, and methanation reformate purification reactions are also exothermic. Central to the thermal efficiency of a reformer is the thermal integra-tion of both the reforming and purification stages. Through careful engineering design, heat can be moved from one part of the process to another, reducing the amount of net heat that must be added or removed.

    Exothermic reactions have better startup and transient response than endothermic reactions as the heat of reaction is immediately available without the time lag associated with adding heat. Despite this advantage, exothermic processes can require complex control strategies in order to control temperatures and avoid overheating catalysts and materials over the entire operating range. Excess heat from exothermic reactions must be dissipated through a cooling system.

    Endothermic reactions require the addition of heat in con-cert with the instantaneous load demand. To some extent, this problem can be lessened by designing reformers with

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    Key Points & Notes high thermal inertia that tend to smooth out the differences between transient peaks and valleys. Nevertheless, heat must be added using burners and complicated control sys-tems that attempt to provide the required heat on demand.

    Typical burner designs use flames or catalytic combustors, with or without a thermal buffer. Burners are usually fueled using the hydrogen-depleted raffinate, the fuel cell anode exhaust stream, a portion of the reformate, or some of the source fuel. Combustion can lead to NOx emissions if the burner temperature exceeds 2700 ºF (1480 ºC).

    Flame Burners

    Flame burners generate heat through the direct combustion of the fuel with air.

    Flame burners can supply the radiant heat of combustion directly to a metal reactor wall. This is a very common ap-proach for high temperature steam reformers, and can also be applied in low temperature reforming systems.

    Flame burning has the advantage of mechanical simplicity, although hot spots can easily occur where the flame contacts the reactor. This can lead to localized overheating and cata-lyst degradation, especially when used in a low temperature application. To overcome this, a thermal buffer can be used as an intermediate heat transfer medium, smoothening out the temperature profile.

    Thermal buffers usually take the form of flue gases or ther-mal fluids, such as heating oils. Oils are applicable only to low-temperature reformer designs. Oils are particularly ad-vantageous since they have high heat capacity, which allows heat to be stored in a small amount of fluid at temperatures significantly lower than that of the flame. This high heat capacity also allows the reformer to operate at a nearly con-stant temperature for optimum efficiency. Heat from an oil buffer can be transferred to the reformer and to the vapor-izer beds through a coiled tube embedded in the vessel walls. Overheating remains a concern, though, as thermal oils begin to degrade above 645 ºF (340 ºC).

    Catalytic Combustors

    Catalytic combustors generate heat through the combustion of fuel with air over a catalyst such as platinum. This proc-ess allows for complete combustion without flames at lower temperatures than would be required without the catalyst. Combustion temperatures can be maintained at tempera-

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    Key Points & Notes tures of 1290 ºF (700 ºC) or less using very lean air/fuel mixtures.

    Like flame burners, catalytic combustors can supply the heat of combustion directly to a metal reactor wall or by way of a thermal buffer. In either case, the lower combustion temperature decreases the chance of overheating, either in the reactor or in the thermal oil.

    Catalytic combustors also offer the advantages of very com-plete fuel combustion, low emissions (less than 5 ppm CO and no NOx), and very effective and responsive temperature control. Response to load changes is very fast, which is ex-cellent where rapid startup and good transient response are required.

    The disadvantages of catalytic combustors are the increased capital cost associated with the catalyst, and the potential for catalyst poisoning in the event of fuel impurities.

    2.1.3 Thermochemical Water Decomposition

    In thermochemical water decomposition, heat alone is used to decompose water. This process is similar to electrolysis, with the difference that all the energy is added as heat and none as electricity. The maximum theoretical efficiency for this process is about 50%.

    Water decomposes directly at very high temperatures (>4500 ºF; 2500 ºC) which is too high to be practical. Instead, chemical reagents are used to decompose water in multi-step chemical reactions in the following sequence:

    1. Oxygen production

    2. Hydrogen production

    3. Materials regeneration

    These reactions take the generic form of:

    AB + H2O + heat A H2 + B O

    A H2 + heat A + H2

    2 BO + heat 2B + O2

    A + B + heat AB

    This is essentially an oxidation-reduction reaction where A and B are chemical compounds that commonly include iron, oxygen or chlorine. This process can take place at about 1290 ºF (700 ºC).

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    Key Points & Notes Selection of the specific chemical compounds for the reaction series depends on such factors as the:

    • amount of energy needed for each step

    • stability of the reactants for each step

    • ability for the products of one step to be easily reacted in the next

    • reaction time

    • reaction yield

    • individual reaction temperature

    • pressure and flow requirements

    • number of steps

    • amount of cleanup required

    • composition of waste products

    In addition, as with any other chemical process, heat recov-ery, mass transfer and materials issues affect the overall system design.

    Thermochemical water decomposition, like electrolysis, is only as renewable and environmentally clean as its energy source. If waste heat is used from other processes, the re-sulting hydrogen increases the overall efficiency of the heat-generating parent process. If heat is obtained by burning fossil fuels, the resulting hydrogen will likely have generated more pollution than would have been by using the source fuel directly.

    2.1.4 Photo Conversion

    Photo conversion is an electrolysis process by which the electricity needed to decompose water into hydrogen and oxygen is generated directly using solar energy.

    Water cannot absorb solar energy directly since it is trans-parent to the required wavelengths. The most common proc-ess uses a series of cells, each with a pair of semi-conductor photoelectrodes immersed in a watery electrolyte and sepa-rated by a membrane that allows ion transfer but prevents gas mixing. The cathode, is made of a “p-type” semiconduc-tor material (that has an excess of positive “holes”). The anode, is made of a “n-type” semiconductor material (that has an excess of electrons). This is similar to a solar cell.

    When the anode is illuminated by sunlight, electrons absorb the light energy and pass through an external circuit to the

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    Key Points & Notes cathode. This creates an “electron-hole” pair. The positive hole subsequently reacts with the water to create positive hydrogen ions and gaseous oxygen. The hydrogen ions pass through the membrane to recombine with the electrons at the cathode, resulting in hydrogen gas. Thus, the overall process results in photoelectrolysis of the water.

    Figure 2-18 Semiconductor Photo Conversion Cell

    A related method uses photochemical catalysts, suspended in alkaline or acidic solutions, as opposed to semiconduc-tors. These catalysts absorb photon energy upon illumina-tion, which creates an electric charge that drives a water-splitting reaction.

    Like any solar energy power generation process, photo con-version can only work in areas of prolonged, concentrated sunlight. Furthermore, this type of system has maximum efficiencies of between 8 and 12%.

    Photobiological Processes

    Light energy can also produce hydrogen through photo-biological processes, using biological systems such as cya-nobacteria (blue-green) algae, photosynthetic algae or eukaryotic (green) algae. In principle, these algae contain hydrogen-metabolizing enzymes that feed off base com-pounds in an anaerobic environment and release hydrogen gas in the process. These algae use compounds such as water, whey, starches, sugar refinery waste, and distillery

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    Key Points & Notes waste as electron donors. Unfortunately, the efficiency of these processes is very low; typically less than 1%.

    2.1.5 Production from Biomass

    Hydrogen can be produced from the decomposition of bio-mass. Biomass comprises all manner of plant and animal material that can be converted into energy. Biomass is or-ganic in nature and can be derived from a variety of sources including residues, wastes and crops. Residues include the materials left over after the harvest of crops or trees. Wastes include those from food processing plants, sewage and mu-nicipal solid waste. Crops include those that can be grown specifically for their energy content, such as soybeans, trees and woody plants, among many others.

    Potential U.S. biomass supplies for energy purposes have been estimated to be the energy equivalent of 7 million barrels of oil per day: 40% from biomass waste resources and 60% from biomass energy crops grown on 80 million acres of excess agricultural lands.

    The appeal of hydrogen production from biomass is the obvi-ous abundance of waste materials and its subsequent poten-tial to supply large amounts of hydrogen. The primary problems are that the process is land and water intensive, and is hampered by the difficulties associated with handling solids. Furthermore, the associated technology is not well developed with the result that it is currently the most expen-sive means of obtaining hydrogen.

    Hydrogen production from biomass takes two primary forms; thermochemical conversion and anaerobic digestion.

    Thermochemical Conversion

    Thermochemical conversion is a multistage process that includes feedstock pre-processing, gasification, gas cleanup, steam reforming, shift reaction and hydrogen separation (or alternatively, methanol synthesis and purification).

    Gasification is at the heart of the thermochemical conversion process. Gasification occurs through the thermal decomposi-tion (or “pyrolysis”) of organic matter with the help of an auxiliary gas such as air, oxygen, or hydrogen to yield pri-marily gaseous products.

    Typically, the feedstock reacts with steam and air (oxygen), at a temperature dictated by the feedstock characteristics, to rapidly yield a mixture of hydrogen, carbon monoxide, car-bon dioxide, water and methane along with hydrogen sulfide and other trace impurities.

    Hydrogen content is determined by the process temperatures and pressures and can constitute about 20% of the gas mixture. A major disadvantage of direct biomass gasification is the co-production of tars, phenolics and acids. Effective

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    Key Points & Notes separation and purification of the decomposition products and efficient gas cleanup for the removal of tars and oils remain as technical challenges.

    Catalysts can be used to increase production efficiencies, reduce tar formation and eliminate the need for oxygen. The process can become even more profitable if undertaken un-der pressure, as the equipment needed becomes smaller and less expensive.

    Anaerobic Digestion

    Anaerobic digestion (or “methane fermentation”) is a multi-stage process in which the waste from one set of organisms is the food for another set. This process takes place in an anaerobic environment, usually in a digester, in the pres-ence of a population of anaerobic bacteria.

    Anaerobic digestion consists of three basic processes. First, cellulose and hemicellulose are broken down by enzymes to form soluble organic compounds. Then the soluble organic materials are converted to hydrogen, carbon dioxide, formate and acetate by acid-producing bacteria. Finally, methane is produced from those substances by methane-producing bacteria.

    The resulting methane gas production is very slow at ambi-ent temperatures, and the digester temperature must be increased to the order of 100 ºF (37 ºC). The resultant gas has medium fuel value, and the concentration of methane is only slightly higher than its carbon dioxide content. As a result of these two factors, anaerobic digestion processes use almost as much energy as they produce, or even more, de-pending on the design of the digester and the operating conditions. Although hydrogen can be obtained from meth-ane through steam reforming, this additional step requires too much energy to be cost effective.

    2.1.6 Industrial Processes

    Hydrogen is produced as a byproduct of some conventional industrial processes. Hydrogen is:

    • a byproduct of chlorine and polyvinylchloride production

    • a by-product of the sodium hydroxide industry

    • produced among other light gases in crude oil refineries

    • produced from coal in coke oven gases

    • emitted in chemical dehydrogenation processes

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    Key Points & Notes Generally these by-product sources have insufficient capac-ity and are too costly to supply large quantities of hydrogen.

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    Key Points & Notes 2.2 Hydrogen Storage If the greatest challenge in hydrogen use is to extract it, the second greatest challenge is how to store it. As seen in Sec-tion 1, hydrogen has the lowest gas density and the second-lowest boiling point of all known substances, making it a challenge to store as either a gas or a liquid. As a gas, it requires very large storage volumes and pressures. As a liquid, it requires a cryogenic storage system.

    Hydrogen’s low density, both as a gas and a liquid, also results in very low energy density. Stated otherwise, a given volume of hydrogen contains less energy than the same volume of other fuels. This also increases the relative storage tank size, as more hydrogen is required to meet a given vehicle’s range requirements. The amount of hydrogen needed for fuel cells is offset somewhat by the fact that it is used more efficiently than when burned in an internal com-bustion engine, so less fuel is required to achieve the same result.

    Despite its low volumetric energy density, hydrogen has the highest energy-to-weight ratio of any fuel. Unfortunately, this weight advantage is usually overshadowed by the high weight of the hydrogen storage tanks and associated equip-ment. Thus, most hydrogen storage systems are considera-bly bulkier and/or heavier than those used for gasoline or diesel fuels.

    For all practical purposes, hydrogen can be stored as either a high-pressure gas, a liquid in cryogenic containers, or a gas chemically bound to certain metals (hydrides). The vol-ume and weight of each of these systems is compared to gasoline, methanol and battery storage systems (each con-taining 990,000 Btu (1,044,500 kJ) of stored energy; equiva-lent to 8.4 gallons of gasoline) in Figure 2-19. Ironically, the best way to store hydrogen is in the form of hydrocarbon fuels although it requires additional systems to extract it.

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    Key Points & Notes

    Figure 2-19 Storage Volume and Weight of Comparative Fuels Each With 990,000 Btu (1,044,500 kJ) of Stored Energy

    2.2.1 High Pressure Gas

    High-pressure gas storage systems are the most common and most highly developed methods of storing hydrogen. Most existing fuel cell vehicles use this form of hydrogen storage.

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    Key Points & Notes High-pressure hydrogen is stored in cylinders, similar to those used for compressed natural gas. Most cylinders have a cylindrically shaped sidewall section with hemispherical end domes, although new conformal designs use multiple cylinders in tandem and distort the cylindrical shape in order to increase the usable volume.

    The terminology“high pressure gas” usually denotes pressures above 3000 psig (207 barg) when used in relation to gas storage systems. However, any gas pressure above 30 psig (2 barg) has the potential to cause serious human injurand should be considered “high” in hum

    y

    an terms.

    Ports in the middle of each end dome allow the gas to flow into and out of the cylinder, with end bosses that control the gas flow. One end boss acts primarily as a plug, although it includes a pressure relief device vent port and may contain pressure and temperature transducers to measure gas con-ditions within the cylinder. The other end boss is a sophisti-cated portal device that includes a solenoid valve, an excess flow valve, a manual isolation valve, a check valve and a pressure relief device vent port.

    Figure 2-20 High Pressure Hydrogen Cylinder Designs

    The solenoid valve is normally closed and isolates the cylin-der whenever the vehicle is shut down. The excess flow valve closes whenever the gas flow leaving the cylinder is too great (such as if a pipe bursts). The manual isolation valve allows the cylinder contents to be either isolated or vented manu-ally in the event of solenoid valve failure. The check valve allows fueling while the solenoid valve is closed. The pres-sure relief devices (at either end of the cylinder) release the cylinder contents when exposed to fire.

    High-pressure gas cylinders must be made of thick-walled, high strength materials and must be very durable. Cylinders are classified into four types according to their materials of construction.

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    Key Points & Notes Desig-nation

    Description % Load Taken by Metal vs. Composite

    Type 1 A cylinder made wholly of steel or aluminum

    100 / 0

    Type 2 A cylinder with a metal line of steel or aluminum and a hoop-wrapped (circumferential) composite overwrap

    55 / 45

    Type 3 A cylinder with a thin metal liner of steel or aluminum and a fully wound composite overwrap

    20 / 80

    Type 4 A cylinder with a plastic liner and a fully wound composite overwrap

    0 / 100

    Table 2-2 High Pressure Gas Cylinder Classifications

    In general, the less metal used, the lower the weight. For this reason, Type 3 cylinders are usually used in hydrogen applications, and Type 4 cylinders will likely gain promi-nence in the future. Specific weights depend on individual manufacturers, but as a point of reference, a 3.5 ft3 (100 L) Type 1 (steel) cylinder weighs about 220 lb (100 kg) a Type 3 (aluminum/composite) cylinder weighs about 143 lb (65 kg), and a Type 4 cylinder weighs about 66 lb (30 kg).

    Figure 2-21: Type 3 Cylinder Construction

    Type 3 cylinders derive most of their strength from the com-posite overwrap that is wound around the inner liner. This composite consists of high-strength fibers (usually carbon) that are wrapped around the cylinder in many layers and glued together by a resin such as epoxy.

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    Key Points & Notes

    Figure 2-22: Composite Cylinder During Manufacturing

    The combination of fibers and resin used in a composite cylinder is extremely strong provided that the fibers and resin remain undamaged. Composite surfaces are less tough than metal surfaces and are more susceptible to physical damage (cuts, abrasion, impacts, etc.) and chemical damage (ammonia, acids, etc.), although they are less susceptible to corrosion.

    Cylinder manufacturers strive to attain the highest storage pressures possible in order to reduce the required storage volume. High-pressure cylinders typically store hydrogen at up to 3600 psig (250 barg) although new designs have been certified for 5000 psig (350 barg) operation. State-of-the-art technology currently under development has exceeded the accepted burst test standard of 23,500 psig (1620 barg) using a 10,000 psig (700 barg) Type 4 cylinder.

    High temperatures from hot ambient conditions, or as a result of compression during fueling, may increase storage pressures by 10% or more. Any gas stored at these high pressures is extremely dangerous and is capable of releasing a gas stream with explosive force or propelling small projec-tiles like a bullet. An unrestrained cylinder could turn into a rocket if the gas were suddenly released through a small opening.

    Cylinders are certified for a specific gas and must survive rigorous tests before being put into service.

    Despite the potential danger, high-pressure cylinders have an excellent safety record. Cylinder designs must conform to

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    Key Points & Notes rigorous standards and survive certification tests that dem-onstrate:

    • no failures during 13,000 pressure cycles to 100% service pressure plus 5000 cycles to 125% service pressure. Some standards also demand 30 cycles to 166% service pressure.

    • no failures (after pre-conditioning) during 5000 pressure cycles to 100% service pressure at 140 ºF (60 ºC) followed by 5000 cycles at –40 ºF (–40 ºC).

    • burst pressure of 2.25 to 3.0 times the service pressure, depending on the standard.

    • impact resistance to flaws, drops and pendulum impact.

    • safe venting of contents when exposed to a bonfire.

    • no fragmentation when exposed to gunfire.

    Some cylinder tests performed on a Type 4 cylinder are illus-trated Figure 2-23.

    During manufacture, each cylinder is subjected to hydro-static and leak tests, and selected cylinders from each lot are subjected to cyclic and burst tests. Cylinders are labeled by the manufacturer as to the construction standard, serial number, service pressure, maximum fill pressure, and end-of-service date. Cylinders that are not subjected to severe external abuse or gas pressures above normal service levels can expect a service life of 15 years or 11,250 fills. Inspec-tions and leak tests are a routine part of cylinder mainte-nance.

    A typical vehicle application uses a series of cylinders mounted on a common manifold. At a service pressure of 3600 psig (250 barg), the fuel storage system weighs nearly four times that of a comparable liquid hydrogen storage system and occupies more than four times the space. When compared to gasoline, the gaseous hydrogen storage system is 15 times greater by volume and 23 times by weight. To put this into perspective, approximately 50% of a transit bus roof must be covered with hydrogen cylinders in order to replace its standard diesel tank.

    Despite this volume, the entire weight of hydrogen fuel is only 90 to 110 lb (40 to 50 kg), which is negligible compared to the weight of the cylinders and associated equipment. Storing gas at an even higher pressure results in a smaller storage volume, but the overall storage weight to hydrogen volume does not change much as the cylinders must be more robust. The current record for the highest hydrogen

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    Key Points & Notes storage by weight ever recorded at 11.3% as achieved using a 5,000 psig (350 barg) Type 4 cylinder.

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    Key Points & Notes

    Figure 2-23: Cylinder Qualification Tests

    Gas compression is an energy intensive process. The higher the end pressure, the greater the amount of energy required. However, the incremental energy required to achieve higher and higher pressures decreases so that the initial compres-sion is the most energy intensive part of the process.

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    Key Points & Notes The energy economy of higher compression levels is counter-balanced by an incremental decrease in gas density at higher pressures, so that further compression packs less hydrogen mass into the cylinders (even if suitable cylinders were to exist). A useful way of understanding the energy cost of compression is as a percentage of the total energy content (LHV) of the hydrogen being stored. In these terms, approxi-mately 5% of the LHV is required to compress the gas to 5000 psig (350 barg). Exact energy usage depends on the flow capacity and efficiency of the compressors used.

    2.2.2 Liquid

    Liquid hydrogen storage systems overcome many of the weight and size problems associated with high-pressure gas storage systems, albeit at cryogenic temperatures.

    Figure 2-24: Automotive Liquid Hydrogen Tank, Sectioned

    Liquid hydrogen can be stored just below its normal boiling point of –424 ºF (–253 ºC; 20 K) at or close to ambient pres-sure in a double-walled, super-insulating tank (or “dewar”). This insulation takes the form of a vacuum jacket, much like in a Thermos bottle. Liquid hydrogen tanks do not need to be as strong as high-pressure gas cylinders although they do need to be adequately robust for automotive use.

    Hydrogen cannot be stored in liquid form indefinitely. All tanks, no matter how good the insulation, allow some heat to transfer from the ambient surroundings. The heat leakage rate depends on the design and size of tank — in this case,

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    Key Points & Notes bigger is better. This heat causes some of the hydrogen to vaporize and the tank pressure to increase. Stationary liquid hydrogen storage tanks are often spherical since this shape offers the smallest surface area for a given volume, and therefore presents the smallest heat transfer area.

    Tanks have a maximum overpressure capacity of about 72 psi (5 bar); if the hydrogen is not consumed as quickly as it vaporizes, the pressure builds to a point where it vents through a pressure relief valve. This vented hydrogen is not only a direct loss of usable fuel, but it also poses a flamma-bility hazard if the vehicle is parked indoors. Provision must be made to vent the hydrogen safely without the potential for accumulation. Current automotive hydrogen tank technology provides a venting (or “boil-off”) rate of about 1 to 2% per day.

    Hydrogen can be drawn from the tank either as a liquid or as gas. When used in an internal combustion engine, liquid hydrogen can be injected directly into the cylinders in order to increase the amount of fuel combusted in each power stroke. When used in a fuel cell engine, gaseous hydrogen can be drawn off at sufficient pressure to feed the power generating reaction.

    Although liquid hydrogen storage systems eliminate the danger associated with high pressures, they introduce dan-gers associated with low temperatures. A severe frostbite hazard exists in association with the liquid hydrogen, its vapors and contact surfaces. Carbon steel exposed to tem-peratures below –22 ºF (–30 ºC), either directly or indirectly, becomes brittle and is susceptible to fracture. Air may liq-uefy on the outside of exposed liquid hydrogen lines or under insulation resulting in an oxygen concentration that poses a fire or explosion hazard if it drips onto combustible materi-als.

    Liquid hydrogen is considerably more dense than gaseous hydrogen but is still much more bulky than gasoline on an equivalent energy basis. Liquid hydrogen storage systems can be four to ten times larger and heavier than an equiva-lent gasoline tank.

    Hydrogen liquefaction is a very energy intensive process due to the extremely low temperatures involved. Liquefaction involves several steps, including:

    1. Compression of hydrogen gas using reciprocating com-pressors; pre-cooling of the compressed gas to liquid ni-trogen temperatures –319 ºF (–195 ºC; 78 K)

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    Key Points & Notes 2. Expansion through turbines

    3. Catalytic conversion to its stable “parahydrogen” form (Section 1.1)

    In total, the energy required for the liquefaction process is the equivalent of up to 40% of the LHV of hydrogen. Once in liquid form, hydrogen is relatively efficient to transport and easy to use. Clearly, to maximize the energy investment paid during liquefaction, it is prudent to store and use the hydro-gen directly as a liquid whenever possible.

    The worst scenario in terms of energy investment is to liq-uefy the hydrogen, transport it in liquid form, re-convert it to a gas, and store in on-board a vehicle as a high pressure gas. This erodes the net available energy twice, once during liquefaction and again during compression, while still being left with the disadvantages of a bulky and heavy on-board gaseous fuel storage system.

    2.2.3 Metal Hydrides

    Rare Earth Hydrides

    Metal hydride storage systems are based on the principle that some metals readily absorb gaseous hydrogen under conditions of high pressure and moderate temperature to form metal hydrides. These metal hydrides release the hy-drogen gas when heated at low pressure and relatively high temperature. In essence, the metals soak up and release hydrogen like a sponge.

    The advantages of metal hydride storage systems revolve around the fact that the hydrogen becomes part of the chemical structure of the metal itself and therefore does not require high pressures or cryogenic temperatures for opera-tion. Since hydrogen is released from the hydride for use at low pressure (and must be released before it can burn rap-idly), hydrides are the most intrinsically safe of all methods of storing hydrogen.

    There are many types of specific metal hydrides, but they are primarily based on metal alloys of magnesium, nickel, iron and titanium. In general, metal hydrides can be divided into those with a low or high hydrogen desorption (release) tem-perature.

    The high temperature hydrides may be less expensive and hold more hydrogen than the low temperature hydrides, but require significant amounts of heat in order to release the hydrogen. Low temperature hydrides can get sufficient heat

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    Key Points & Notes from an engine, but high temperature hydrides require an external source of heat.

    Figure 2-25: Metal Hydride Vehicle Storage Tank, Sectioned

    The low desorption temperatures associated with some hy-drides can be a problem since the gas releases too readily at ambient conditions. To overcome this, low temperature hy-drides need to be pressurized, increasing the complexity of the process. The characteristics of typical metal hydrides are summarized in Table 2-3.

    The main disadvantage of metal hydride storage systems is not so much the temperatures and pressures needed to release the hydrogen, but rather their low mass energy den-sity. Even the best metal hydrides contain only 8% hydrogen by weight and therefore tend to be very heavy and expensive. Metal hydride storage systems can be up to 30 times heavier and ten times larger than a gasoline tank with the same energy content.

    Another disadvantage of metal hydride storage systems is that they must be charged with only very pure hydrogen or they become contaminated with a corresponding loss of capacity. Oxygen and water are prime culprits as they

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    Key Points & Notes chemically adsorb onto the metal surface displacing poten-tial hydrogen bonds. The storage capacity lost through con-tamination can to some extent be reactivated with heat.

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    Key Points & Notes

    Low Temperature High Temperature Characteristic

    Ti2Ni-H2,5 FeTi-H2 VH-VH2 LaNi5-H6,7 Mg2Cu-H3 Mg2Ni-H4 Mg-H

    Alloy mass that can absorb hydrogen

    1.61% 1.87% 1.92% 1.55% 2.67% 3.71% 8.25%

    Hydride mass equivalent to the energy in 0.264 gal (1 L) of gasoline

    342 lb 155 kg

    295 lb 134 kg

    286 lb 130 kg

    355 lb 161 kg

    No Data

    149 lb 67.5 kg

    79 lb 35 kg

    Alloy mass necessary to accumulate 5.5 lb (2.5 kg) of hydrogen

    478 lb 217 kg

    414 lb 188 kg

    401 lb 182 kg

    496 lb 225 kg

    No Data

    209 lb 95 kg

    110 lb 50 kg

    Desorption temperature at 145 psig (10 barg)

    93 ºF 34 ºC 307 K

    125 ºF52 ºC 325 K

    127 ºF53 ºC 326 K

    163 ºF 73 ºC 346 K

    604 ºF 318 ºC 591 K

    662 ºF 350 ºC 623 K

    683 ºF 362 ºC 635 K

    Desorption temperature at 22 psig (1.5 barg)

    26 ºF –3 ºC 270 K

    44 ºF 7 ºC 280 K

    59 ºF 15 ºC 288 K

    70 ºF 21 ºC 294 K

    480 ºF 245 ºC 522 K

    512 ºF 267 ºC 540 K

    565 ºF 296 ºC 569 K

    Charging Easy No Data

    No Data

    Very Difficult

    No Data

    Difficult Very Difficult

    Safety Safe No Data

    No Data

    No Data

    Highly Flam-mable

    Safe Highly Flam-mable

    Table 2-3 Metal Hydride Characteristics

    A further problem associated with metal hydrides relates to their structure. Metal hydrides are typically produced in a granular or powder form so that they have a large surface for gas storage. These particles are susceptible to attrition, which both reduces their effectiveness and may plug relief systems or other piping.

    No specific metal hydride has outstanding performance on all accounts (high absorption capacity, high density, low heat requirement and low cost). In some cases, a mixture of low and high temperature hydrides can be used to maintain some of the advantages inherent to each while at the same time introducing some of disadvantages of each.

    Alkaline Earth Hydrides

    A recent hydride variation that offers some advantages over previous methods involves the use of pelletized sodium, potassium or lithium compounds. These hydride compounds react with water to release hydrogen without the addition of heat.

    The most commercially developed process involves the use of sodium hydroxide (NaOH), which is abundantly available as

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